Fabrication of nano Ag encapsulated on ZnO/Fe2V4O13hybrid-heterojunction for photodecomposition of Methyl Orange

Novel Ag encapsulated nanocompositesZnO/Fe 2 V 4 O 13 (AZF) were synthesized with various wt% of the silver (from 1.0 to 2.5 wt% of Ag) by photo-deposition method using UV-A light. The nanostructure of the AZF was explored by various characterization techniques. Surface functional group was con�rmed by FT-IR spectra, the crystalline nature of the material revealed by XRD patterns. Furthermore, surface morphology and optical properties of the composites were analyzed by SEM, HR-TEM UV-DRS and PL respectively. The photoactivity was tested against Methyl Orange (MO) degradation under UV light. The stability of the catalyst was con�rmed by reusability measurements. Suitable degradation pathway was proposed based on intermediates obtained during degradation analyzed by GC-MS. Trapping experiments con�rmed that super oxide radical anion (O 2·–) has been consider as a most active species for this degradation process. Mineralization is con�rmed by measurements of chemical oxygen demand (COD).


Introduction
Dye pollutants released by the textile industries are becoming a signi cant source of pollution for the environment. It is estimated that about 150 tons a day will be released into the aquatic system worldwide.The discharge of this dye e uent is a major cause of non-esthetic degradation in the water body and these dyes are also resistant to aerobic degradation and can be converted into carcinogenic aromatic amines under anaerobic conditions [1][2][3][4]. Advanced oxidation processes (AOPs) producing hydroxyl radical ( • OH) as one of the most effective oxidants were considered promising techniques.
Photocatalytic processes based on the application of semiconductors as a photocatalyst for the degradation of toxic organic contaminants to in aqueous phase have been widely studied among various AOPs [5][6][7][8][9].In the photocatalytic activity of certain dye molecules, zinc oxide (ZnO) is documented to be more effective than TiO 2 [10,11]. ZnO is becoming a good choice for photocatalyst applications. ZnO is found to be ine cient alternative to TiO 2 since its photodegradation mechanism has been proved to be almost same. However, ZnO has a wide band gap of 3.37 eV, which ultimately limits in phtotodegradation due to rapid electron-hole recombination. Different attempts have been made to enhance the effective charge separation in ZnO to address this limitation, thus strengthening its photocatalytic ability. Encapsulation of noble metal nanocomposite has been proven e cacious due to strong surface plasma resonance (SPR) of noble metal which extendedvisible light absorption with charge separation. Recently, Nanosilver (AgNPs) has been paying intense attentiondue to its high performance of photocatalytic ability when it combined with semiconductors. Ag has been identi ed as the best elements to encapsulate with ZnO, due to its high solubility [12].

XRD
The average crystallite size of 2 wt% of AZF was 28.5 nm.

BET surface area
The pore structure and surface area of the prepared Ag-ZnO/Fe 2 V 4 O 13 was analyzed using N 2 absorptiondesorption isotherms are shown in Fig. 3. The Ag-ZnO/Fe 2 V 4 O 13 is type II isotherm at IUPAC level [27]and the distribution of pore size is given in the inset of Fig. 3. The BET surface area and pore volume of Ag-ZnO/Fe 2 V 4 O 13 are given in Table 1.
0.9043 0.1044 461.86 16.53 0.0089 S BET = BET surface area, V p = total pore volume, D p = average uniform pore size distribution, S micro = surface area of micropores, V micro = pore volume of micropores.   Figure 5 shows the EDX recorded from the selected area, which reveals that the presence of Zn, Fe, V, Ag, and O in the catalyst. The presence of these elements in Ag-ZnO/Fe 2 V 4 O 13 was also con rmed by elemental color mapping. The different color areas in Fig. 6

Photoluminescence (PL) emission spectra
The effective suppression of photogenerated charge carriers and the transfer of the photogenerated e -/h + were investigated by photoluminescence (PL) emission spectra [29]. Figure 9 shows the PL spectra of the prepared ZnO (Fig. 9a) and 2 wt% Ag-ZnO/Fe 2 V 4 O 13 (Fig. 9b) [30].The maximum intensity shows the higher e -/h + recombination and resultslow photocatalytic activity [31,32].The lowest intensity shows that the well suppression of e -/h + recombination and results higher photocatalytic activity [33].

Primary analysis
The photocatalytic behavior of nanocomposite Ag-ZnO/Fe 2 V 4 O 13 with 1, 1.5, 2, and 2.5 wt percent of Ag loading was assessed in terms of MO degradation.Controlledexperimentswere conducted under different reaction conditions (Fig. 10). The dye/ZnO/UV-A process underwent 68% degradation in 90 min (curve a). 3.9 Effect of pH pH is the important parameters for the application of industrial point of view.By adjusting the pH of the MO solution, the effect of pH on the MO photodegradation was studied. Fig. S2demonstrates that pH has an important effect on the rate of photodegradation and decolorization. The maximum degradation and decolorization of MO is observed at pH 7. Above pH 7, the rate of degradation and decolorization decreases. Effect of catalyst loading (Fig. S3) and initial dye concentration (Fig. S4) are discussed in supporting information's. Figure 11 shows the stability of the catalyst for the degradation of MO. In the rst run, approximately 99% of MO degradation achieved. The same catalyst was again reused for further runs. All the remaining cycles gave almost 98.5% of degradation in 90 min. Hence, the 2 wt% Ag-ZnO/Fe 2 V 4 O 13 is stable, recoverable and reusable.

Mineralization studies 3.11.1 GC-MS analysis
Sometimes intermediates are more hazardous than starting materials, so it is necessary to analyze the intermediates for the degradation process. An attempt has been made to nd out the degradation intermediates of MO photodegradation with 2wt% Ag-ZnO/Fe 2 V 4 O 13 (AZF) hybrid-heterojunction/UV process. GC-MS studied performed with the solutions obtained after 30 and 60 minutes of irradiation and predicted a degradation pathway for MO by AZF based on the m/z ratio, retention time and molecular weight(Scheme 1). For these identi ed intermediates, molecular ion and fragmentation peak values are stated inTable 2.Although, in photocatalytic degradation of azo dyes, it was expected that the cleavage of azo bond take place rst, however, formation of compounds observed (D1 and D2)with azo groups at the retention time of 19.796 and 18.533 min, respectively. Hydroxyl radicals were thus considered to be the most reactive species for degradation, the compound D1 undergoes azo link cleavage and replacement of sulfonic acid group by hydroxyl group through the repetitive attack of • OH radicals produced intermediates N′-methylbenzene-1,4-diamine (Compound I) and 4-aminophenol (D4). The intermediate product D2 further undergoes C-N cleavage produced D3 which on further undergoes azo link cleavage produced 4-aminophenol (D4). Finally, it is expected that the compound D4 and compound I would be mineralized to CO 2 , water and mineral acids [34,35].

FT-IR spectral analysis
The early adsorption of the dye under dark by 2 wt% Ag-ZnO/Fe 2 V 4 O 13 nanocomposite is 33.4%, although complete degradation occurred at 90 min irradiation. The experiments were carried out to determine whether the adsorbed dye molecules had been degraded completely. Comparisons made with FT-IR spectra of the fresh dye and catalyst (( Fig. S5a and S5b), and dye adsorbed composite before and after irradiations ( Fig. S5c and S5d). The characteristic bands of MO (Fig. S5a) are observed at 1604, 1366, and 1042 cm -1 due to N = N stretching, C-N bond vibrations, and S = O bond of MO, respectively [36].When compared Fig. S5a and Fig. S5c, the characteristic MO dye peaks are observed in the dye adsorbed catalyst. However, upon irradiation,ie after complete degradation, the FT-IR spectrum of the composite (Fig. S5d) resembles with fresh catalyst (Fig. S5b)   with respect to time, and nally almost completely disappeared. From these observations we conclude that the degradation of the dye with respect to time [37].Moreover, no new peakswere observed during irradiation, indicating that MO was degraded gradually and intermediates do not absorb at analytical wavelengths. The color of the suspension changed from the orange to colorless (inset of Fig. 12).

Chemical oxygen demand (COD) measurements
The mineralization of the dye further con rmed with reduction of COD values. Under optimum condition, COD measurements were made and the percentageof COD reduction of thedye at different times of UV-A light irradiation is given in Table 3. Percentages of COD reduction increases with respect to irradiation time reveals that the mineralization of the dye.  (7) facilitate the degradation e ciently with3 g L -1 catalyst loading. The stability of the catalyst was observed by multiple runs of the catalyst. Almost 99% of degradation was observed for all the ve runs. GC-MS reveals that the formation of three azo compounds (D1, D2 and D3) and 4-aminophneol (D4) as intermediates during degradation process. Trapping experiments con rms that the super oxide radical anion (O 2 •-) has been considered as a most active species for this degradation process. The complete mineralization was con rmed by COD measurement. A suitable degradation mechanism is also proposed.

Declarations Con icts of interest
There is no con ict of interest declare